This invention relates, in general, to a method for making semiconductor power devices, and more particularly, to a process for making a power MOSFET semiconductor device.
Typically, power semiconductor manufacturers fabricate p-channel power MOSFET devices with polysilicon gate layers that are doped with an n-type dopant. The polysilicon gate layers typically are formed and doped with a n-type dopant early in the manufacturing process (e.g., before forming the n-type high voltage (NHV) regions). With this process, the polysilicon gate layers must then be protected during the formation of the p-type source regions to prevent the incorporation of p-type dopant into the gate layers.
With n-type polysilicon gate layers, p-channel devices have higher threshold voltages than n-channel devices made with equivalent processing. This is because of the work function difference between the n-type polysilicon gate layer and the underlying p-type substrate. To reduce the threshold voltage of p-channel devices, some power semiconductor manufacturers have resorted to sub-micron processing and p-channel shape redesign while maintaining n-type polysilicon gate layers. However, these techniques require significant capital investment and complex processing steps.
Manufacturers of large scale integrate (LSI) low voltage CMOS structures have found that using boron-doped gate regions for p-channel devices reduces the threshold voltage difference between p-channel and n-channel devices. However, because boron diffuses at a much faster rate than n-type dopants such as phosphorous, manufacturers must exercise great care to prevent boron from diffusing from the polysilicon gate region through the gate oxide layer into the underlying channel region. An unstable device can result when boron is present in the channel region under the gate oxide. To reduce boron diffusion, U.S. Pat. No. 5,189,504 shows a process where inert impurities such as carbon or nitrogen are incorporated with boron into the polysilicon gate layer during polysilicon deposition. This process has a disadvantage because a manufacturer must control the concentrations of the various constituents, which adds to process complexity. Thus, there exists a need for a p-channel power MOSFET device that has a lower threshold voltage, that exhibits stable device characteristics, and that uses less complex processing techniques.